This invention relates generally to a thermocouple used in a gasification process and, more particularly, to the use of a new sapphire reinforced outer protection tube to extend the useful life of thermocouples used in a gasification process.
In high temperature gasification processes, a hot partial oxidation gas is produced from hydrocarbonaceous fuels, for example coal. In these processes, the hydrocarbonaceous fuels are reacted with a reactive oxygen-containing gas, such as air or oxygen, in a gasification reactor to obtain the hot partial oxidation gas.
In a typical gasification process, the hot partial oxidation gas will substantially comprise H2, CO, and at least one gas from the group H2O, CO2, H2S, COS, NH3, N2, Ar, along with particulate carbon, ash, and/or molten slag typically containing species such as SiO2, Al2O3, and the oxides and oxysulfides of metals such as Fe and Ca.
The hot partial oxidation gas in the gasification reactor will commonly be at a temperature ranging from 1,700xc2x0 to 3,000xc2x0 F., and more typically in the range of about 2,000xc2x0 to 2,800xc2x0 F., and at a pressure commonly in the range of about 1 to about 250 atmospheres, and more typically in the range of about 15 to 150 atmospheres.
Thermocouples are commonly used for measuring temperature in these high temperature processes. The thermocouples can be used to measure the temperature in the gasification reactor. They may also be used to measure the temperature in downstream process steps in which the effluent is cooled and particulate and gaseous contaminants are removed.
Thermocouples are pairs of wires of dissimilar metals which are connected at both ends. The content of the wires must be sufficiently dissimilar to allow for a difference in electrical potential between them. Except for the ends, the two wires are electrically insulated from each other. The electrical insulation is commonly provided by a tube of insulating material having two non-intersecting holes passing lengthwise through the tube. Typical insulating materials include high temperature, high purity ceramics, such as alumina.
When the two junctions of the wires are at different temperatures, a difference in electrical potential exists between them. The difference in electrical potential and therefore the difference in temperature can be measured by a voltage measuring instrument placed in the thermocouple circuit or alternatively by a voltage measuring instrument that is sent signals by a transmitter placed in the thermocouple circuit.
The choice of dissimilar metals used for the thermocouple will vary depending on, among other things, the expected temperature range to be measured. For instance, one type of thermocouple commonly employed under the conditions present in a gasification reactor has one wire that contains platinum and about 30% rhodium and a second wire that contains platinum and about 6% rhodium. Other pairs of metals are used for different temperature ranges.
One problem apparent with the use of thermocouples in the environment present in a gasification process, particularly the environment present in the gasification reactor, is the relatively short lifespan of the thermocouples. The relatively short lifespan is due in part to the extremely high temperatures and corrosive atmosphere that prevails during the operation of the gasification reactor. An unprotected thermocouple left in this environment is quickly attacked and rendered useless. Such attack can be most severe when the thermocouple comes into contact with molten slag present in the reactor.
To alleviate this problem, thermocouples are commonly inserted into a refractory thermowell disposed along the outer wall of a gasification reactor or other exterior process surface. The refractory thermowells would include barriers of chrome-magnesia, high chrome, or similar slag resistant materials, and may incorporate other refractory and non-refractory materials such as Al2O3, MgO, and stainless steel.
When used in a gasification reactor, the thermowell may be introduced by passing it through an opening in the outer wall of the reactor pressure vessel. The thermowell may then pass through a corresponding opening in a refractory material, or series of refractory materials, commonly used to line the inner surface of the reactor pressure vessel. The thermowell may extend into the open space of the reactor or it may be set back at a slight distance from the interior of the reactor.
Unfortunately, positioning the thermocouple inside a thermowell has not provided a complete solution. Over time, molten slag will breach the thermowell. The breach is commonly due to the effects of erosion and corrosion as well as thermal and/or mechanical shocks and stresses. However, the breach may also be due, totally or in part, to an inherent fault in the thermowell. The breach, typically small initially, allows molten slag to enter the thermowell where it can come in contact with the thermocouple, rendering it useless.
It would therefore be beneficial to have a means to increase the lifespan of thermocouples used in a gasification process.
In some gasification processes, such as described in pending U.S. patent application Ser. No. 09/106,133, herein incorporated by reference, use of a sapphire envelope enclosing at least a portion of the thermocouple has been described to increase the life of the thermocouple. In the application pending as Ser. No. 09/106,133, the sapphire envelope is in the form of a sapphire sheath fitted over the tip of thermocouple. That apparatus may also comprise a thermowell, with the sapphire envelope being provided inside the thermowell.
In some embodiments of the present invention, it is contemplated that sapphire or other corundum may be added to an outer protection tube to extend the operating periods of thermocouples used in gasification processes without the need for a thermowell.
In one embodiment of the present invention there is disclosed an apparatus including a thermocouple for measuring the temperature in a gasification process, the improvement comprising a sapphire reinforced outer protection tube arranged about the thermocouple and enclosing at least a portion of the thermocouple. Th embodiment may further include an inner protection tube inside the outer protection tube, the inner protection tube being receptive of the thermocouple. The inner protection tube may include alumina or sapphire. The sapphire used for the outer protection tube may be structural-grade non optical fiber, and the fiber reinforces a composite, the composite and sapphire reinforcement defining the outer protection tube.
In some embodiments the outer protection tube may be attached to a support tube. In such embodiments the outer protection tube may be tapered to fit snugly inside a distal end of the support tube. The outer protection tube and support tube may fully enclose the thermocouple.
In some embodiments the outer protection tube is inserted directly into a gasification stream without a thermowell.
The outer protection tube may be molded around an inner protection tube, and the inner protection tube is receptive of a thermocouple. The inner protection tube may include alumina or pure (synthetic) sapphire.
Some embodiments include a sapphire sheath removably arranged about a distal end of the thermocouple, the sapphire including synthetic sapphire. The sapphire sheath may include an open end portion and a plug portion, with both the open end portion and the plug portions including synthetic sapphire. In this embodiment, distal ends of a pair of thermocouple wires may be bent to a radially outward biased configuration to facilitate a snug fit between the sapphire sheath and the thermocouple. This embodiment may further include an inner protection tube inside the outer protection tube, the inner protection tube being receptive of the thermocouple and sapphire sheath. The inner protection tube may include alumina or synthetic sapphire. In a variation of this embodiment, platinum foil is wrapped around the thermocouple to provide a snug fit between the sapphire sheath and the thermocouple.
There is also disclosed a thermocouple system for use in a gasification system including a thermocouple, the thermocouple including a pair of wires of dissimilar metal joined together at one end by a hot junction and at the other end by a cold junction but otherwise electrically insulated by an insulating tube; a thermocouple inner protection tube receptive of the pair of wires and the insulating tube; and a thermocouple outer protection tube, wherein the outer protection tube includes sapphire. In this embodiment the outer protection tube may further include a sapphire-reinforced ceramic integrally molded around the inner protection tube. The inner protection tube may include sapphire. A support tube may be connected to the outer protection tube, the support tube extending into a gasification reactor. The temperatures to be measured range from about 1,300xc2x0 F. to about 3,000xc2x0 F.
There is also disclosed a method of measuring temperatures in a gasification process including the steps of: providing a thermocouple comprising a pair of wires of dissimilar metal joined together at one end by a hot junction and at the other end by a cold junction but otherwise electrically insulated from each other by an insulating tube; providing an outer protection tube comprising sapphire connected to a support tube; inserting the outer protection tube and support tube into a gasification reactor; and inserting the thermocouple into the outer protection tube.
There is also disclosed a method of constructing a sapphire-reinforced outer protection tube including the steps of: providing a mold; providing an inner protection tube; inserting the inner protection tube into the mold; adding sapphire fiber to a composite to create a sapphire-reinforced composite; pouring the sapphire-reinforced composite into the mold where at least a portion of the inner protection tube is surrounded by the sapphire-reinforced composite; and curing the composite.